1. Field of the Invention
This invention relates generally to offshore floating structures, and, more particularly to an offshore floating platform having an ocean thermal energy conversion system.
2. Background Art
Demand for energy worldwide and the need for alternate energy sources is increasing significantly. Ocean Thermal Energy Conversion (OTEC) is a viable alternate energy source which has been in the development stage for the past three to four decades. A great amount of energy is available in deep-ocean environments with temperature difference between an upper surface layer and a lower deep-ocean layer may be within a maximum range of approximately 25° C. in localized offshore locations near equatorial waters. However, the technology is not in commercial operation due to the large capital cost. Advances in heat-exchanger material, cold-water pumps and working-fluid are areas in which extensive research has been done to make OTEC a successful system. However, none of these improvements have made OTEC technology attractive for cost effective commercialization. The present OTEC system provides a feasible OTEC system for about 100 MW power plants with significant differences and advantages over conventional systems. The present OTEC system reduces the capital cost significantly and makes OTEC technology commercial feasible. The present OTEC system which is supported in new types of floating vessels, or “floaters” achieves cost efficiency and reduces the capital cost of the OTEC system compared to conventional OTEC systems. Both the new OTEC system and the supporting floater system are presented herein.
One of the popular renewable solar energy sources is the ocean using the temperature differences that exists between warm water at the upper surface layer and cold water in the lower deep-ocean layer. The process that uses this natural thermal gradient is known as Ocean Thermal Energy Conversion (OTEC). The deep ocean is a natural storage basin for solar energy that could be available to tap all day around. The minimum temperature difference between the warm surface-water and the deep cold-water required is 20° centigrade in order for the OTEC system to produce a significant amount of electric power. Thus, the ocean is the vast source of renewable energy, with the potential to produce billions of watts of electric power. The feasible locations that have sufficient temperature differences existing between the surface and deep-ocean layers are in deep ocean areas near the equator.
A conventional OTEC closed cycle power plant engine design typically uses standard heat-engine cycles which are used in power plants wherein the heat from the burning fuel is converted into electrical power. The primary components of the OTEC power plants are: the heat exchangers, the cold water pipe, the working fluid, the supporting ocean platform, and the underwater transmission lines. Ammonia with a low boiling point is typically used for the working fluid of the OTEC closed cycle system. An evaporator is used to vaporize the working fluid with the help of the pumped warm surface seawater. The turbine is rotated by expending the heat-energy of the ammonia vapor that comes out from the evaporator. The ammonia that flows out from the turbine is cycled through the condenser to cool the ammonia vapor causing it to become liquid. The cold water obtained from the deep ocean layer is used for the condenser function. The thermodynamic cycle continues and stabilizes to produce electricity without the need of burning an external fuel to produce the heat-energy in the OTEC cycle. The OTEC system is classified as an open-cycle and a hybrid-cycle. Different working fluid alternates have been studied by researchers to improve efficiency.
The conventional OTEC power plant is typically supported on a floating barge which is operated in deepwater locations where the required temperature difference is available. Although there is no need for external fuel for the OTEC engine and it is obtained free from ocean thermal energy, the success of the OTEC power plant predominantly depends on the capital cost of the plant and the cost per kilowatt hour of the energy produced. Previous studies show that OTEC could be more economical if designed for large power output. In order to produce a large power output, the conventional OTEC power plant requires a very large diameter cold-water intake pipeline extended to a depth of approximately 3000 ft in the water and is supported at the top by the vessel. The pipe is to be designed for wave environmental loads. The vortex induced oscillations also could be a significant problem due to underwater ocean current. Secondly, the heat exchanger, especially the condenser, required for the large OTEC plants is expensive due to its size and weight. Another problem is that a large amount of water needs to be moved from the deep-ocean layer to the surface and handled on the vessel to be used in the condenser. This also makes the conventional OTEC system inefficient and cost sensitive.
If OTEC could become cost competitive with the rest of alternate energy resources, then billions of watts of electric power could be produced from the OTEC system. Bringing the OTEC system capital cost down is a real challenge faced by engineers and scientists. The present invention provides a method that significantly reduces the OTEC system cost, and makes the OTEC system feasible.
Several costs are involved in a typical conventional OTEC system OTEC system. The platform, installation, equipment system, underwater cable, cable installation, maintenance, management, marketing, local infrastructures are some of the cost divisions built into an OTEC project that would effect the total capital cost. The vessel and equipment costs are predominant in the conventional system, and are significantly reduced by the innovative design of the present system.
Referring now to Table 1, below, the cost for a conventional ship-shaped floating unit having with a conventional OTEC system with a 100 MW output is shown. The costs are approximate estimations, and a more detailed cost could be obtained from a detailed analysis for a specific location. The total cost obtained from Table 1 is $420 million US dollars. The capital cost of the 100 W conventional power plant depends a great deal on the cost of the heat exchanger unit. Thus, it would be desirable to improve the design to reduce the cost of the heat exchanger. Even if a cost effective cold water pipe of composite material were utilized, the cost is still not significantly controllable for large units. Small units are not beneficial to the location, and not cost effective due to the size of the supporting vessel. Because of the large size of the cold water pipe, it is difficult to handle from the surface floating vessel in harsh environments. If buoy-type connect and disconnect features are added to the cold water pipe, that cost is also added to the design. Again, the vessel designed to support the conventional OTEC is sensitive to storms and harsh environments. That poses large operational costs to the OTEC system in a deep sea. Thus, the overall $420 million US dollar capital cost for a 100 MW power plant is not attractive for commercialization.
TABLE 1Sl. NoOTEC System Components% CostMillion US $1Platform Vessel Structure17%702Turbine Generator & Pumps18%753Evaporator21%904Condenser21%905Cold-Water Pipe2.5% 106Subsidiary Equipment3.5% 157Working Fluid2.5% 108Underwater Cable 5%209Installation Cost 5%2010Operational & Maintenance Cost 5%20Total Cost100% 420
Heat transfer is the major engineering task to be involved in the success of any efficient OTEC power plant. Conventional OTEC systems perform the heat transfer effort on the top of the platform deck. The amount of water needed to cool the working fluid is enormous. Conventional OTEC systems typically utilize a very large diameter cold water pipe to bring the cold water from the depth of about 3,000 ft. to the free surface where the condenser is located to condense the vapor or gas into a liquid after it leaves the turbine, and the warm surface water is used at the evaporator in the gas heating process to produce the vapor or gas which is transported to the turbine. The large diameter cold-water intake pipe extending from a depth of approximately 3,000 ft. in the water is supported at the top by the vessel, and must be designed for environmental wave loads. The cold water pipe is massive and is subject to vortex induced oscillations due to underwater ocean current and huge stresses at the joint between the cold water pipe and the OTEC platform resulting from a combination of severe weather, wave action, and the length, diameter, and mass of the cold water pipe. Thus, in the conventional OTEC system, an enormous amount of heavy equipment is used for handling large volumes of water on the surface of the floating vessel, which requires an increase in the vessel size and consequently a very high capital cost.
This requirement for large capital costs has contributed to the lack commercial operation of the OTEC technology. Advances in heat-exchanger material, cold-water pumps and working-fluid are areas in which extensive research has been done to make OTEC a successful system. However, none of these improvements have made OTEC technology attractive for cost effective commercialization.
Prueitt, U.S. Published Patent Application 2007/0289303 discloses a system for heat transfer for OTEC (Ocean Thermal Energy Conversion), wherein rather than transferring huge quantities of cold water from deep in the ocean to the surface to provide a heat sink for a heat engine or for desalination, the invention provides a method of using small masses of low-boiling-point fluids to absorb heat in a heat exchanger near the ocean surface using the latent heat of evaporation and then depositing the latent heat of condensation in a deep ocean heat exchanger, using the cold seawater as a heat sink. The condensed liquid is pumped back to the ocean surface. The heat engine (turbine) and generator can be at the ocean surface, or it can be in deep ocean water. By using a fluid that transfers heat by evaporation and condensation, much larger quantities of heat can be moved per kilogram of fluid than can be transferred by moving the same mass of seawater.
Prueitt, U.S. Published Patent Application 2009/0077969 discloses heat transfer methods for OTEC (Ocean Thermal Energy Conversion) and desalination which produce fresh water from seawater on both the boiler side and the condenser side of an OTEC power plant. Part of the warm ocean surface water is evaporated, and its vapor transfers heat to the working-fluid boiler as the vapor condenses. The condensation of the vapor provides fresh water. On the condenser side, the condensation of the working-fluid vapor from the turbine in the condenser releases heat that evaporates seawater that runs down the outside of the condenser surfaces. The vapor from the seawater is condensed by a heat exchanger that uses input from colder seawater. As the cold seawater accepts heat from the condensing vapor, it becomes slightly warmer and provides the source of seawater that accepts heat from the condenser. The condensing vapor on the heat exchanger becomes fresh water that is drawn out as potable water. To provide additional fresh water, a multi-stage desalination unit uses the warm water discharge and the cold-water discharge from the OTEC plant to provide a temperature gradient that causes evaporation and condensation in each stage of the unit.
Howard, U.S. Published Patent Application 2009/0178722 discloses a system for relieving the stress on an Ocean Thermal Energy Conversion (OTEC) cold water pipe which includes a slidable joint that couples the OTEC cold water pipe to a surge tank at an opening in the surge tank. The system may further include a first flotation device that is coupled to the OTEC cold water pipe below the surge tank, and a second flotation device that is coupled to the OTEC cold water pipe within the surge tank.